1. Field of the Invention
The present invention relates to an apparatus and method for fumigating soil under raised planting beds that are covered with plastic mulch.
2. Description of the Related Art
Soilborne pests and pathogens, including weed propagules, nematodes, insects, fungi, bacteria and certain other agents, can be limiting factors in the production of crops. One of the principal strategies used by growers of high-value horticultural crops to combat these organisms is to disinfest soil prior to planting, using pesticides or other physical or biological methods. Soil fumigants are the most effective soil disinfestation chemicals, and methyl bromide (MB) is the most important soil fumigant chemical used by growers around the world. It is a broad-spectrum pesticide with excellent activity against most potential soil pests (Stapleton et al., California Agriculture, Vol. 54 (6), 42–45, December 2000).
In production systems where polyethylene plastic is used as a mulch, preplant application of chemical fumigants is the principal method employed for control of soilborne pests. In Florida, for example, approximately 4.9 million Kg of methyl bromide were applied to 20,000 ha of tomato (Lycopersicon esculentrum Mill.) during 1990 to control soilborne pests (Anonymous, USDA NAPIAP, April, 1993; Anonymous, Fla. Dep. Agric. Consum. Serv., 1993). Increased social and legislative pressure to restrict the use of chemical fumigants has created the impetus to evaluate alternative approaches for management of soilborne diseases which minimize their impact.
Chellemi et al. (Plant Disease, Volume 78 (12) 1167–1172, December 1994) studied the effects of soil solarization using a photoselective polyethylene film, soil fumigation, and combinations of the two on survival of four soilborne plant pathogens in field plots in Northern Florida, and found that soil solarization alone resulted in a significant decrease in the density of Phytophthora nicotianae and Pseudomonas solanacearum down to a depth of about 25 and 15 cm. respectively. Significant reductions in the density of Fusarium oxysporum radicis-lycopersici and F.o. lycopersici following soil solarization occurred only in the upper 5 cm of soil. Fumigation significantly reduced populations of Phytophthora nicotianae, F.o. radicis-lycospersici, and F.o. lycopersici down to a depth of 35 cm. The effect of fumigation on P. solanacearum was highly variable. Additional reductions in the density of P. solanacearum were achieved when solarization was combined with fumigation. Synergistic effects between soil solarization and chemical or organic amendments that resulted in additional reductions of pathogen populations have been observed for other soilborne pests (Frank et al., Crop Prot., Vol. 5, 199–202, 1986; Gamliel et al., Plant Dis., Vol. 77, 886–891, 1993; Gamliel et al, Phytopathology, Vol. 83, 899–905, 1993).
Various systems and apparatus have been devised for injecting chemical fumigants into soil or spraying fumigants onto the soil surface and then sealing the fumigants using tarpaulins or foams. However, problems associated with fumigant emission from the soil have resulted in a loss of their effectiveness, disruption to the environment, and have posed heath risks to workers in the field.
Other methods for applying fumigants include drip irrigation, where a water soluble formulation is applied to soil with irrigation water through the same irrigation systems that are later used to irrigate the crops. The amount of water used to apply the fumigants, application rates, and soil conditions during application determine the success of these fumigants in controlling soil pathogens and weeds.
U.S. Pat. No. 3,640,234 (Carroll et al., Feb. 8, 1972) discloses a fumigant injection apparatus which forms, fills and seals capsules of volatile fumigants at least 3 feet underground allowing the fumigant to diffuse through the soil and control pests and prevent fumigant escape into the atmosphere.
U.S. Pat. No. 3,964,405 (Haglund et al., Jun. 22, 1976) discloses the use of spot penetration injectors, special furrowing equipment, and utilization of tooth or chisel applicators generally attached to cultivators to deeply place fumigants into the soil at about 14–24 inches below ground level.
U.S. Pat. No. 6,167,821 (Beggs, Jan. 2, 2001) discloses a device for applying fumigant below the surface of the soil and includes a soil leveling apparatus, and a soil packer. The apparatus further includes a cover crop seed planter located behind the soil leveling apparatus. The device includes a conventional fumigant applicator which is capable of applying fumigant to the soil at a depth of about 2 to 12 inches below the top surface of the soil. The fumigating portion of the device consists of multiple chisel plows including an elongated tine with a conventional chisel tip. A stainless steel fumigant supply conduit with a spray nozzle at the end is connected to the tine so that the nozzle is approximately halfway down the tine length. When the chisel is lowered to a depth of about 10 to 12 inches, the sprayer is at a depth of about 4 to 8 inches allowing spray to reach the bottom of the chisel.
Miller et al.(Down to Earth, 18–20, Fall 1967) describe a soil injection knife for applying fumigants to bedded land. The blade of the chisel knife is designed so that it sweeps down and back from the tool bar forcing stalks and other crop debris down and off the tip of the blade as it passes through the soil. The knives enter the side of a bed about three inches above the bottom of the furrow. The tips of the knives are about 12 inches apart during fumigant application. This requires approximately 15½ inches insertion into the bed. The tips of the blades are then about 6 inches off center of the bed. Each knife contains drill holes for spraying fumigant.
While various methods and apparatus are used for applying fumigants to soil, there remains a need for methods and apparatus for fumigation which inject fumigants under raised planting beds that have been previously covered with plastic mulch. The present invention provides a device for fumigation under existing raised, plastic-mulched beds without the use of drip irrigation which solves some of the problems of the prior art.